BRCA1 (B1) is a breast and ovarian tumor suppressor. Its best known function is in the repair of double strand breaks (DSB) by homologous recombination (HR). It can also function in other types of DNA damage repair (cf below), as well as in DNA damage checkpoints. However, there is limited in depth knowledge about any of the purported functions for B1or how it acts as a tumor suppressor. To address this problem, we purified B1-containing protein complexes from a human cell line and analyzed the complexes by mass spectrometry (MS). Our hope was that by better defining the B1 protein partner repertoire we might be able to better understand known B1 functions or potentially hypothesize new B1 functions based on the known functions of identified interactors. In this analysis, we repeatedly identified a protein called TONSL (aka NFKBIL2) as a B1 interactor. This protein has been suggested to function in the repair of collapsed replication forks. Recently, our group has shown that B1 functions in the repair of stalled replication forks. Given the similar suggested function for both proteins, and based on our preliminary analysis of the B1-TONSL interaction, we hypothesize that B1 recruits TONSL to collapsed replication forks to aid in the repair of DSB breaks arising at these sites. The four specific aims of this proposal wil focus on this hypothesis. The goal of specific aim 1 will be to perform a structure function analysis on the B1-TONSL interaction as a means of assessing, genetically, the hypothesis that B1 recruits TONSL to collapsed forks and that the B1- TONSL complex is involved in repair of these structures. The goal of aim 2 will be to understand an observation from our preliminary data. We find that after UV damage, B1 localizes at UV damage sites in virtually all S/G2 cells. However, TONSL co-localizes with B1 at these sites in only some of these cells. The goal of aim 2 is to determine why this is the case, which could shed light on how B1-TONSL complexes function. The goal of aim 3 is to search for and, if feasible, identify other members of the B1-TONSL subcomplex, by affinity purification and MS. By doing this we hope to identify subunits with known biochemical functions and then assess the contributions of these properties to B1-TONSL function and vice versa. This approach could further illuminate aspects of the function of this complex at collapsed forks and/or suggest new hypotheses for the function of this complex. The goal of aim 4 is to determine, with help from collaborators, whether TONSL and/or any other member of the B1-TONSL complex is a product of a breast cancer suppressor gene by searching for relevant mutations in whole genome sequence libraries of breast tumors. This work will be supervised by the candidate's sponsor in a program composed of laboratory-based research training leavened by attendance at relevant scientific meetings. Ideally, success in this effort will contribute to a better understanding of B1 function and how loss of B1 function leads to breast and/or ovarian cancer.